Combustion of hydrocarbon-based fuels in electrical power stations and engines produces flue or exhaust gas that contains, in large part, relatively benign nitrogen (N2), water vapor (H2O), and carbon dioxide (CO2). However, the flue and exhaust gases also contain, in relatively small part, noxious and toxic substances, such as carbon monoxide (CO) from incomplete combustion, hydrocarbons (HC) from un-burnt fuel, nitrogen oxides (NOx) from excessive combustion temperatures, and particulate matter (mostly soot). To mitigate the environmental impact of flue and exhaust gas released into the atmosphere, it is desirable to eliminate or reduce the amount of the undesirable components, preferably by a process that, in turn, does not generate other noxious or toxic substances.
Typically, exhaust gases from lean burn gas engines and flue gases from electrical power stations have a net oxidizing effect due to the high proportion of oxygen that is provided to ensure adequate combustion of the hydrocarbon fuel. In such gases, one of the most burdensome components to remove is NOx, which includes nitric oxide (NO) and nitrogen dioxide (NO2). The reduction of NOx to N2 is particularly problematic because the exhaust gas contains enough oxygen to favor oxidative reactions instead of reduction. Notwithstanding, NOx can be reduced by a process commonly known as Selective Catalytic Reduction (SCR). An SCR process involves the conversion of NOx, in the presence of a catalyst and with the aid of a reducing agent, such as ammonia, into elemental nitrogen (N2) and water. In an SCR process, a gaseous reductant such as ammonia is added to an exhaust gas stream prior to contacting the exhaust gas with the SCR catalyst. The reductant is absorbed onto the catalyst and the NOx reduction reaction takes place as the gases pass through or over the catalyzed substrate. The chemical equation for stoichiometric SCR reactions using ammonia is:4NO+4NH3+O2→4N2+6H2O2NO2+4NH3+O2→3N2+6H2ONO+NO2+2NH3→2N2+3H2O
Zeolites are a type of molecular sieve having a three dimensional framework made of aluminosilicates. Zeolites having certain structures, such as AEI, AFX, BEA, CHA and MOR are known to be useful as SCR catalysts. Such zeolites have a molecularly porous crystalline or pseudo-crystalline structure constructed primarily of alumina and silica. The catalytic performance of these zeolites can be improved by incorporating an extra-framework metal, for example, by a cationic exchange wherein a portion of ionic species existing on the surface of the framework is replaced by metal cations, such Cu2+. Typically, higher metal concentrations can correspond to higher catalytic performance.
CHA zeolites containing extra-framework iron are known to be stable. However, these catalysts have a slow transient response when Fe or Mn is present as an extra-framework metal. However, the use of LTA, a small pore zeolite, with either Fe or Mn as an extra-framework metal provides the same stability and hydrocarbon poisoning benefits as found with the use of CHA zeolites containing extra-framework metals, along with the transient response of a large pore zeolite.